1. Field of the Invention
The present invention relates to the preparation of adipic acid by hydrocarboxylation of pentenic acids, and, more especially, to the preparation of adipic acid by reacting water and carbon monoxide with at least one pentenic acid.
2. Description of the Prior Art
European Patent Application No. 188,209 describes a process for the preparation of straight-chain dicarboxylic acids, in particular adipic acid, by reacting unsaturated monocarboxylic acids, notably pent-3-enoic acid, with carbon monoxide and water in the presence of a rhodium-based catalyst and an iodine-containing promoter, the reaction being carried out in a solvent such as methylene chloride at a temperature of 100.degree. to 240.degree. C. and under a total pressure of between 14 and 240 atm; a temperature ranging from 150.degree. to 180.degree. C. and a total pressure ranging from 24 to 40 atmospheres are considered to be preferred.
The selection of the solvent and the water content of the reaction mixture are critical according to this '209 application, such constraints, however, militating against the applicability of the process on an industrial scale.
U.S. Pat. No. 3,579,552 describes the use of a rhodium-based catalyst and an iodine-containing promoter in reactions for the hydrocarboxylation of olefins and certain of the derivatives thereof. However, the proportion of straight-chain acid produced remains insufficient.
U.S. Pat. No. 3,579,551 describes the use of an iridium-based catalyst and an iodine-containing promoter in reactions for the hydrocarboxylation of olefins and certain of the derivatives thereof. However, the activity of such a catalyst system remains inadequate.
Thus, serious need continues to exist for an improved catalyst system which simultaneously provides enhanced activity and enhanced selectivity for adipic acid while at the same time avoiding the above disadvantages and drawbacks to date characterizing the state of the art.